Tropospheric scatter is a communication technique in which a transmitter transmits radio signals toward the stratosphere, and a receiver receives a portion of the signal that is scattered in the stratosphere. Tropospheric scatter (often referred to as “troposcatter”) enables long-distance, non line-of-sight communication, without the need for manmade repeaters.
Various troposcatter communication techniques are known in the art. For example, U.S. Pat. No. 7,711,030, whose disclosure is incorporated herein by reference, describes techniques for spatial-multiplexed tropospheric scatter communications. A disclosed method comprises transmitting a training signal from each antenna of a base station to each of a plurality of client devices utilizing tropospheric scatter. Each of the client devices analyzes each training signal to generate channel characterization data, and transmits the channel characterization data back to the base station utilizing tropospheric scatter. The channel characterization data is stored for each of the plurality of client devices. Data to be transmitted to each of the client devices is received, and the data is precoded using the channel characterization data associated with each respective client device to generate precoded data signals for each antenna of the base station. The precoded data signals are transmitted through each antenna of the base station to each respective client device.
Accurate antenna alignment is an important feature in troposcatter systems. Various troposcatter antenna alignment schemes are known in the art. For example, U.S. Pat. No. 4,347,514, whose disclosure is incorporated herein by reference, describes an antenna alignment scheme in a troposcatter system. Antenna alignment is accomplished by nutating the antenna receiver beam with a sinusoidal nutation signal to induce envelope modulation on the received signal, cross-correlating the envelope of the received signal with the nutating signal, and aligning the average angle of the antenna receiver beam into the position where the crosscorrelation goes to zero.
As another example, U.S. Pat. No. 4,170,011, whose disclosure is incorporated herein by reference, describes an azimuthal alignment method for troposcatter system antennas, which is achieved by centering on the great circle, the combined pattern of intercepting beams from two troposcatter antennas. The combined antenna pattern is determined to be centered on and symmetric about the great circle when the Doppler spectrum on a received signal is symmetric about zero Doppler shift.